1. Field of the Invention
This invention relates to a key switch for an electronic piano, which is configured to detect a motion of a key or a hammer that pivotally moves in accordance with depression of the key, as key depression information.
2. Description of the Related Art
Conventionally, the above-mentioned type of key switch for an electronic piano has been disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2007-52357. The key switch is of a three-contact type that detects a motion of a key in three stages. The key switch is comprised of a dome-shaped rubber switch fixedly secured to the lower surface of the key and a switch board disposed below the rubber switch. The rubber switch has cylindrical first to third switch elements extending vertically downward from the top wall and arranged in a front-rear direction (lengthwise direction) of the key. The first switch element is the longest of all the first to third switch elements, and the third switch element is the shortest of all. The first to third switch elements have lower end portions formed with respective first to third movable contacts. On the other hand, the switch board has first to third stationary contacts in a manner opposed to the first to third movable contacts, respectively.
As the key pivotally moves during key depression, the rubber switch of the key switch is pressed by the key to be deformed. The first to third movable contacts are sequentially brought into contact with the first to third stationary contacts, respectively, in accordance with the deformation of the rubber switch, whereby respective ON signals are output.
FIG. 10 shows the arrangement of the stationary contacts of a general three-contact type key switch in the prior art. As shown in FIG. 10, each of the first to third stationary contacts CS1 to CS3 is comprised of a common contact (hatched contact) and a non-common contact (unhatched contact). The common contacts of the respective first to third stationary contacts CS1 to CS3 have the same reference potential e.g. by being grounded, and the non-common contacts have a potential different from the reference potential. The common contacts and the non-common contacts are each formed e.g. of carbon printed on the switch board, and have a rectangular shape linearly extending in the front-rear direction (left-right direction as viewed in FIG. 10). Each of the common contacts and the associated one of the non-common contacts are closely opposed to each other in the left-right direction (vertical direction as viewed in FIG. 10). Further, the three common contacts and the three non-common contacts are both arranged side by side in a state adjacent to each other in the front-rear direction.
With this arrangement, when the key is depressed, first to third movable contacts cm1 to cm3 each formed e.g. of carbon are brought into contact with the common contacts and the non-common contacts in a manner bridging the respective pairs of the common contact and the non-common contact, to make the common contacts and the non-common contacts conductive, whereby ON signals are output.
The present assignee has already proposed the above-described type of key switch for an electronic piano e.g. in Japanese Laid-Open Patent Publication (Kokai) No. H08-44361. The key switch is comprised of a switch board having first and second stationary contacts arranged thereon side by side in a lengthwise direction of a hammer, switch bodies each formed of a soft elastic material and mounted on the switch board in a manner opposed to the hammer, and first and second movable contacts arranged side by side on each switch body in a manner opposed to the first and second stationary contacts thereof, respectively. The distance between the first movable contact and the first stationary contact is set to be smaller than that between the second movable contact and the second stationary contact. Further, on the surface of the switch body, there is formed a single pressure-receiving protuberance extending in a direction orthogonal to the lengthwise direction of the hammer.
According to this key switch, when the hammer pivotally moves in accordance with key depression and comes into contact with the pressure-receiving protuberance of the switch body, the switch body is pressed via the pressure-receiving protuberance by the hammer, whereby the switch body is compressed and deformed. When the switch body is deformed, the first movable contact and the first stationary contact opposed to each other with the shorter distance therebetween are brought into contact with each other first, whereby a first ON signal is output. Then, as the hammer further pivotally moves, the switch body pivotally moves about the first movable contact held in contact with the first stationary contact, whereby the second movable contact is brought into contact with the second stationary contact and a second ON signal is output.
The first and second ON signals are sequentially output to a tone generation controller. The tone generation controller determines, based on the presence or absence of the first and second ON signals, whether or not a key has been depressed, and identifies an associated key number when a depressed key has been depressed. Further, the tone generation controller determines a key depression speed based on a difference in output timing between the first and second ON signals and controls tone generation of the electronic piano according to these results.
In the conventional key switch constructed as above, the relative position of the movable contact to the stationary contact when brought into contact with the stationary contact (hereinafter referred to as “the contact position of the movable contact”) is apt to shift in the front-rear direction (i.e. in the lengthwise direction of the key) e.g. due to variation in the operation (deformation) of the rubber switch or positional shift that occurs in printing of the stationary contacts on the switch board. However, in the conventional key switch, the first to third stationary contacts CS1 to CS3 has the three rectangular common contacts and the three rectangular non-common contacts located close to each other in the front-rear direction.
For this reason, when the contact position of the movable contact shifts in the front-rear direction and the movable contact deviates even slightly from the stationary contact, the movable contact can be erroneously brought into contact with an adjacent stationary contact, causing erroneous detection. For example, when the first movable contact cm1 deviates from its original position (indicated by a solid line) corresponding to the stationary contact CS1 to a position (indicated by a broken line) shifted toward the second stationary contact CS2 to come into contact with a non-common contact of the second stationary contact CS2, an ON signal which indicates that the second stationary contact CS2 is in the ON state is erroneously output.
In order not only to achieve stabilization of ON signals from a key switch but also to reduce the amount of heat generated by the key switch, it is preferred that the electric resistance of a movable contact formed e.g. of carbon is held as low as possible. Further, in order to reduce the electric resistance, it is effective to increase the diameter of the movable contact, for example. However, when the movable contact of the conventional key switch has an increased diameter, it is more likely that the movable contact is brought into contact with an adjacent stationary contact due to shift of its contact position, and therefore the magnitude of the diameter of the movable contact has to be limited.
Further, as a solution to the above problem, it can be envisaged e.g. to increase the length of the common contact of each stationary contact and that of the non-common contact of the same or increase the space between the stationary contacts. However, since the entire length of the key switch of the three-contact type is originally larger than that of a key switch of the two-contact type, the above-mentioned method further increases the entire length of the key switch, which causes an increase in the size of the key switch.
There is another problem. As described above, according to the conventional technique, the key switch of the two-contact type, which has the first and second movable and stationary contacts, makes it possible to reliably bring the two movable contacts into contact with the respective associated stationary contacts to thereby achieve stable switching operation. On the other hand, the key switch of the three-contact type, which has the first to third movable and stationary contacts, has recently been used in an electronic piano so as to acquire more detailed key depression information and realize a piano tone closer to a tone generated by an acoustic piano. However, when the conventional technique is applied to the key switch of the three-contact type, there is a fear that stable switching operation cannot be ensured for reasons mentioned below, and therefore there remains room to be improved in this point.
In the key switch of the three-contact type, the arrangement length of the movable contacts (i.e. the distance from the first movable contact to the third movable contact) in the lengthwise direction of the hammer is larger than in the key switch of the two-contact type. For this reason, even when the switch body is pressed via the single pressure-receiving protuberance and is pivotally moved about the first movable contact held in contact with the first stationary contact, contact pressure between the third movable contact distant from the first movable contact and the third stationary contact sometimes becomes insufficient. This sometimes makes it impossible to bring the third movable contact into secure contact with the third stationary contact, which can cause a faulty operation.
As a solution to this problem, it can be envisaged e.g. to dispose the pressure-receiving protuberance closer to the third movable contact. With this arrangement, however, the first movable contact having already been in contact with the first stationary contact is apt to move up from the first stationary contact during halfway through a pivotal motion of the hammer, which makes it difficult to maintain the contact state. Therefore, also in this case, there is a fear that a faulty operation can be caused. Further, it can be envisaged to dispose the first to third movable and stationary contacts closer to each other in the lengthwise direction of the hammer so as to avoid the above-mentioned contact failure between the contacts. In this case, however, difference in output timing between the ON signals is reduced, which causes degradation of accuracy in detecting the hammer motion.